Process for purifying melamine-containing ammonia

ABSTRACT

A process for purifying gaseous melamine containing ammonia is provided. The method comprises partly condensing the gaseous melamine-containing ammonia to form melamine in liquid NH 3  and purified NH 3  gas. The melamine and purified NH 3  gas are taken off.

CROSS REFERENCE TO RELATED APPLICATION

This application is a National Phase Patent Application of International Application No. PCT/EP01/12691, filed on Nov. 2, 2001, which claims priority of Austrian Patent Application No. A 1888/2000, filed Nov. 8, 2000.

FIELD OF THE INVENTION

The application relates to a process for purifying gaseous melamine-containing ammonia. The inventive process is suitable in particular for purifying ammonia gases originating from high-pressure melamine plants using anhydrous melamine workup.

BACKGROUND

In high-pressure processes for preparing melamine, urea is converted to melamine in an endothermic liquid-phase reaction. After removal of NH₃ and CO₂, a melamine melt under high pressure is obtained which is then solidified in what are called wet workup processes by quenching with water. In what are called the dry processes, as described, for example, in U.S. Pat. No. 4,565,867, WO 95/01345, WO 97/20826 and WO 99/38852, the melamine melt is solidified by quenching with ammonia, by expanding the NH₃-saturated melamine melt or a melamine/NH₃ suspension at a temperature just above the melting point of melamine, by sublimation with subsequent desublimation or by cooling in a fluidized bed. However, in these processes, sometimes very large amounts of gaseous NH₃ are produced which, depending on pressure and temperature of the gas produced, comprise greater or lesser amounts of melamine. This already CO₂-free gas can be returned to the melamine plant only in part and sometimes only with difficulties. The melamine present in the NH₃ gas causes difficulties and faults, in particular in the compressors, if the NH₃ gas needs to be compressed for liquification. A need therefore exists to free melamine-containing NH₃ gas from melamine in a simple process.

SUMMARY OF THE INVENTION

The invention is directed to a method comprising partly condensing the melamine-containing gaseous NH₃. This produces firstly melamine-containing liquid NH₃ and, secondly, purified melamine-free NH₃ gas which is separated off from liquid NH₃.

More particularly the invention relates to a process for purifying melamine-containing gaseous NH₃, which comprises partly condensing melamine-containing gaseous NH₃ by cooling, the melamine accumulating in the liquid NH₃ and the purified NH₃ gas being taken off.

DESCRIPTION OF THE DRAWING

A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawing, wherein:

FIG. 1 is a schematic view of an exemplary plant in accordance with the invention.

DETAILED DESCRIPTION

The melamine-containing gaseous NH₃ can be cooled using any cooling device, for example in a cooling column or by means of liquid NH₃, liquid NH₃ being mixed with the NH₃ to be purified, for example by spraying liquid NH₃ into the NH₃ to be purified or by passing the NH₃ to be purified through liquid NH₃. Preferably, the melamine-containing gaseous NH₃ is passed through liquid NH₃, in which case it cools and partly condenses. The resultant purified NH₃ gas is taken off and can if appropriate be condensed, and the melamine-enriched liquid NH₃ is discharged. The amount of liquid NH₃ discharged is preferably replaced by the condensing NH₃ gas, but it can also be replaced by feeding fresh NH₃.

The melamine-containing NH₃ gas to be purified is already CO₂-free and preferably originates from dry melamine processes as set forth above, for instance from processes for solidifying liquid or gaseous melamine under NH₃ pressure or a melamine/NH₃ suspension using liquid supercritical or gaseous NH₃. In the dry melamine processes the solidification is performed, for example, by quenching with NH₃, with NH₃ and liquid or gaseous melamine being sprayed into a quencher. In an exemplary embodiment, the melamine-containing NH₃ gas to be purified originates from the fluidized bed of a plant for solidifying melamine, into which the liquid or gaseous melamine is introduced into a fluidized bed made up of solid melamine, or solid inert particles and solid melamine, and maintained using NH₃ gas.

Preferably, the process is carried out continuously. The gaseous melamine-containing NH₃ is, for example, partly condensed using an air- or water-cooled cooler or via direct contact with liquid NH₃. Preferably, the gaseous, melamine-forming NH₃ here is passed through liquid NH₃, with melamine precipitating or dissolving and accumulating in the liquid NH₃. Depending on from which plant for the dry melamine workup the gas to be purified comes, the gas can be of varying pressure and varying temperature and, as a function thereof, differing melamine content. The lower limit of temperature is preferably just above the respective condensation temperature at the respective pressure, and the upper limit at the operating temperatures of the plant section from which the NH₃ to be purified originates.

If the melamine-containing NH₃ originates, for example, from a process for solidifying gaseous melamine using, or in the presence of, ammonia, the pressure is, for example, about 1-20 barand about 1.5-15 bar, respectively, and the temperature is, for example, about 290-520° C. If the melamine-containing NH₃ originates, for example, from a process for solidifying liquid melamine, lower temperatures down to room temperature are also possible, preferably from about 100° C. to just below the pressure-dependent melting point of melamine, for example from about 100 to 340° C., more preferably from about 200 to 320° C. Higher pressures are also possible up to about 500 bar, preferably from about 5 to 250 bar, more preferably from about 10 to 100 bar.

Pressure and temperature in the partial condensation must be chosen in such a manner here that the NH₃ can be condensed using the coolant available. For example, when water is used as the coolant, the pressure shall not be less than 10 bar, in accordance with the ammonia condensation curve.

The process can be preferably and particularly simply carried out in such a manner that the melamine-containing gaseous NH₃ is passed through liquid NH₃ and partly condensed. The NH₃ gas thus purified can then be taken off and the melamine-enriched liquid NH₃ can be discharged. The amount of melamine-containing liquid NH₃ discharged is continuously replaced by condensing NH₃. However, it is also possible to feed in part fresh liquid NH₃. The partial condensation of the gaseous NH₃ can take place either directly on its introduction into the liquid NH₃, or else by condensation of the purified NH₃ continuously forming, for example using a downstream cooler.

On passing the NH₃ to be purified through the liquid NH₃, melamine-enriched liquid NH₃ is formed which, depending on pressure and temperature, contains greater or lesser amounts of melamine. It can be saturated with melamine or subsaturated, or else solid melamine can already have formed in other words, melamine-containing liquid NH₃ is to be taken to mean not only a solution, but also a suspension, of melamine in NH₃.

In a continuous process in which the melamine-enriched NH₃ is continuously discharged via an overflow, the melamine-enriched liquid NH₃ is customarily recirculated to a suitable section of the melamine preparation process. This can be the reactor or a subsequent separator. Advantageously, the recirculation is into a device in which liquid melamine is allowed to stand under ammonia pressure (“aging”) or into a device in which already-solidified melamine is allowed to stand under ammonia pressure (“tempering”).

Preferably, the melamine-containing liquid NH₃ is recirculated to a plant in which liquid melamine is quenched (solidified) under expansion using liquid ammonia or the gaseous ammonia formed on expansion of the liquid ammonia. Preference is likewise given to recirculating the melamine-containing liquid NH₃ to a plant in which a mixture of gaseous NH₃ and gaseous melamine is quenched and solidified using ammonia.

In another embodiment, the melamine-containing liquid NH₃ is recirculated to a fluidized bed that comprises solid melamine, or solid melamine and solid inert matter, and is maintained by NH₃ gas.

A possible plant for carrying out the inventive process is shown diagrammatically in FIG. 1. In the figure are shown the following:

-   (1) fluidized bed, -   (2) pressure vessel, -   (3) cooler, -   (4) intermediate vessel, -   (5) pump, -   (6) melamine-enriched liquid NH₃, -   (7) gaseous melamine-free NH₃, -   (8) gaseous melamine-containing NH₃.

EXAMPLE 1

A melamine-containing gaseous NH₃ stream 8 of 424 kg/h coming from the fluidized bed 1 of a melamine solidification plant is passed, at a temperature of 280° C. and a pressure of 10 bar, from the bottom through the liquid NH₃ in a pressure vessel 2 equipped with a cooler 3 which is filled with liquid NH₃. A portion of the NH₃ condenses and the melamine separates out in the liquid NH₃ 6. To maintain a constant liquid level in the pressure vessel, 384 kg/h of liquid melamine-containing NH₃ 6 are discharged and recirculated to the melamine plant. The purified NH₃ gas 7 vaporizing in the pressure vessel partly condenses in the attached cooler, and the non-condensed portion is taken off as pure NH₃ gas 7 at a rate of 40 kg/h.

EXAMPLES 2-4

In a similar manner to example 1, melamine-containing NH₃ gas was purified at various pressures and temperatures.

The values for Examples 1 to 3 are listed in Table 1.

TABLE 1 NH₃ to be Exam- Temp. Pressure purified Recirculated ple (° C.) (bar) (kg/h) NH₃ (kg/h) Pure NH₃ (kg/h) 1 280 10 424 384 40 2 280 40 740 650 87 3 340 10 380 340 40 

1. A process for purifying melamine-containing gaseous NH₃ comprising: partly condensing melamine-containing gaseous NH₃ to obtain melamine enriched in liquid NH₃ and purified NH₃ gas; and removing the purified NH₃ gas.
 2. The process as claimed in claim 1, wherein: the melamine-containing gaseous NH₃ is passed through liquid NH₃, being cooled and partly condensed to form the purified NH₃ gas and melamine enriched liquid NH₃, the purified NH₃ gas is taken off, and melamine-enriched liquid NH₃ is discharged.
 3. The process as claimed in claim 2, wherein the melamine-enriched liquid NH₃ contains the melamine in dissolved form.
 4. The process as claimed in claim 2, wherein the melamine-enriched liquid NH₃ contains solid melamine.
 5. The process as claimed in claim 2, wherein the melamine-enriched liquid NH₃ is recirculated to a plant for solidifying liquid or gaseous melamine.
 6. The process as claimed in claim 2, wherein the melamine-enriched liquid NH₃ is recirculated to a fluidized bed that comprises solid melamine or a combination of solid melamine and solid inert matter, and is maintained using NH₃ gas.
 7. The process as claimed in claim 1, wherein the melamine-containing NH₃ gas originates from a process for solidifying liquid melamine under NH₃ pressure using liquid, supercritical or gaseous ammonia.
 8. The process as claimed in claim 1, wherein the melamine-containing NH₃ gas originates from a process for solidifying gaseous melamine under NH₃ pressure using liquid, supercritical or gaseous NH₃.
 9. The process as claimed in claim 1, wherein the melamine-containing NH₃ gas originates from the fluidized bed of a plant for solidifying liquid melamine.
 10. The process as claimed in claim 1, wherein the melamine-containing NH₃ gas originates from a fluidized bed of a plant for solidifying gaseous melamine. 